test: add regression test for #7790 (#17473)

Closes #7790

Author: nicolasstucki

tests/pos/i7790.scala

@@ -0,0 +1,9 @@
+// https://github.com/lampepfl/dotty/issues/7790
+trait Foo:
+  given Int = 10
+  def map(f: Int ?=> Int) = f
+  def map(f: Int ?=> String) = f
+
+@main def Test =
+  val m: Foo = ???
+  m.map((x: Int) ?=> x)